1. Field of the Invention
The present invention relates to a linear direct current motor commonly used for moving an object to be moved with high accuracy in, for example, a motion mechanism such as a machine tool or industrial robot.
2. Description of the Prior Art
FIG. 1 shows a drive unit containing a linear direct current motor of the prior apt. Furthermore, this drive unit has a guide unit for guiding an object added to a linear direct current motor.
As shown in the drawing, this drive unit has a long base member 1 and moving object 2 which moves along said base member 1. More specifically, a plurality of rollers (not shown) are provided on moving object 2, and these rollers roll over a track (not shown) formed along the lengthwise direction in base member 1.
On the other hand, the linear direct current motor which composes the above-mentioned guide unit together with a drive unit is composed in the manner described below.
As shown in the drawing, said linear direct current motor is composed of a primary side, equipped with a large number of armature coils 7 arranged in a row in the lengthwise direction of base member 1 on said base member 1, and a secondary side, having a field magnet (not shown) attached to the bottom surface of moving object 2 to as to oppose each of said armature coils 7. Said field magnet is magnetized so that a plurality of N and S magnetic poles are alternately arranged along the direction in which moving object 2 is to move, namely the lengthwise direction of base member 1.
Overhang 1a is formed on one side of base member 1, and a linear scale 4 is provided on said overhang 1a roughly over its entire length. Reflecting portions and non-reflecting portions are formed alternately and precisely on this linear scale 4 in its lengthwise direction. In contrast, small bracket 2a is provided on the side of moving body 2, and light emitting element 5a and light receiving element 5b are mounted on said small bracket 2a.
In the drive unit of said constitution, by supplying a prescribed current to armature coils 7, thrust is produced based on Fleming's left hand rule between the primary and secondary sides. For example, if base member 1 to which the primary side is coupled, is taken to be the stationary side, moving body 2 integrated into a single unit with the secondary side is moved by this thrust.
In addition, the above-mentioned linear scale 4 is used for detection of the position of the secondary side with respect to the primary side. Namely, light is irradiated onto linear scale 4 by light emitting element 5a accompanying movement of the secondary side, and reflected light from each reflecting potion of linear scale 4 is received by light receiving element 5b to obtain pulse signals which are then counted.
However, a position to serve as a reference must be established during counting of pulse signals. The following constitution is commonly employed for that purpose.
As shown in FIG. 2, light blocking plate 10 is installed on base member 1 at the position established to be the reference. A transmission type photosensor 11 is then attached to moving object 2 in order to detect this light blocking plate 10. Photosensor 11 is equipped with a light emitting element and light receiving element. A detection signal is obtained by interrupting the output from this light receiving element based on illuminating light emitted from said light emitting element being blocked by light blocking plate 10. The above-mentioned pulse signals are then counted using this detection signal as the reference position signal.
As is clear from the above, photosensor 11 and light blocking plate 10 are provided in order to obtain a reference position signal for relative movement of the primary side and secondary side in the linear direct current motor of the prior art. Since the space occupied by this photosensor 11 and light blocking plate 10 is relatively large, said direct current linear motor of the prior art has problems left to be solved in terms of attempting to make the size of this linear direct current motor more compact.